As petroleum fuels become scarcer, alternate fuels and more efficient methods of utilizing existing fuels for transportation are needed. When a vehicle, such as an automobile, is operated at an optimum power, its internal combustion engine can be relatively efficient and can have a theoretical thermal efficiency of about 25%. In normal use, i.e., some climbing hills, stopping, starting and accelerating, the actual efficiency realized is considerably less. Delivery, postal, and maintenance vehicles as well as other vehicles in severe stop-and-go situations may experience very low efficiencies. In the quest for greater efficiency, alternative drive systems and power sources have been investigated.
Because electric drive systems can be efficient, and can derive their energy from alternate fuels, the electric automobile is one alternative which has been studied in some depth. However, storage batteries are so bulky and heavy that until batteries with greater capacity and less weight are developed, it does not present a feasible alternative for any type of medium or long distance travel. Moreover, electric batteries have the disadvantage of having to be re-charged for a relatively long period of time in comparison to the time to re-fuel an internal combustion engine.
A hybride combination of a battery-electric drive system and an internal combustion engine can increase the range and usefulness of the electric drive system without the need for large or heavy batteries or long recharge periods. A hybrid electric-internal combustion engine system can also recoup some of the energy otherwise wasted by an internal combustion engine vehicle when idling or operating at low load levels. For example, when the hybrid is stopped or operating at low road-load levels, the electrical system can convert excess mechanical power from the engine into electrical energy and store it by functioning as a generator and charging storage batteries. Then, during a period of high road load levels, the electrical system can return the stored energy to the drive system thru a motor driven by the batteries. In the hybrid system described, this is achieved by operating the internal combustion engine at a relatively constant power and speed, preferably near its point of greatest efficiency, while the electrical system either stores excess available energy or returns stored energy to the driven system to supply a deficit.
In normal driving, the vehicle is called upon to climb and descend hills, stop and start, and accelerate and brake, all of which combine to put a wide range of power demands on the power plant. If only an internal combustion engine were used, it would be called upon to deliver a great range of power and operate over a wide range of speeds.
But in the hybrid electric-internal combustion engine drive system described, the internal combustion engine is called upon to deliver only a relatively even amount of power under all these conditions, because the electrical system stores the excess or supplies any deficit. Under these optimum conditions, the thermal efficiency of the hybrid engine is always about 25% which is significantly better than the average efficiency realized in a conventional internal combustion engine vehicle.
For internal combustion engines, there is, of course, a preferred speed and power level at which to operate the engine. Herein the preferred speed and power level selected was that at which the engine would provide the vehicle in which it is mounted with the greatest mileage per given quantity of liquid petroleum fuel. However, other criteria could be used in making the selection such as the speed and power level at which the engine produces the least amount of pollutants, etc.
This hybridization produces many beneficial results. Compared to a conventional internal combustion engine vehicle, the hybrid can: employ a smaller engine; provide greater fuel economy; produce lower exhaust emissions and furnish longer engine life with less maintenance. These benefits accrue from the use of the electrical system as a load leveler, i.e., to store or release energy as required and thereby maintain engine power at a point of greatest efficiency or lowest emissions. Operation at relatively constant power and engine speed also provides a more uniform engine operating condition which reduces engine wear.
Compared to an all electric vehicle, the hybrid can: employ smaller batteries with less capacity; reduce or eliminate battery recharging by the conventional way, and provide increased vehicle range and usefulness. These benefits similarly accrue from the use of the electrical system as a load leveler, i.e., the batteries are only required to furnish tractive power to the drive system for short periods of time. Benefits also derive from proper design of the system so that the batteries are kept recharged, more or less, by the engine and motor generator whenever the vehicle is stopped or road load requirements are less than engine power output. Range is naturally extended because all or a large part of the energy required for normal operation can now be supplied by the petroleum fueled engine instead of the batteries.
Moreover, in the past, in order to maintain an internal combustion engine in a hybrid system at a nearly constant power level and to control transfer of power between the engine and motor-generator, relatively complex engine or electric motor controls were necessary. See for example U.S. Pat. No. 3,732,751. The present invention eliminates the need for such complex controls.
The present invention, then, uses a relatively small internal combustion engine. One, which when operating at a preferred speed, produces roughly the average power which the vehicle will require in normal use. This invention further uses a direct current motor-generator with a peak power capability at least equal to the difference between average and peak system power requirements and with a no-load speed comparable to the preferred speed of the internal combustion engine. At motor-generator speeds below the no-load speed, this motor-generator draws current from the batteries and functions as a motor. At speeds above the no-load speed, the motor-generator furnishes current to the batteries and functions as a generator.
In this manner, speed variations of the motor-generator, above and below the no load speed, control the load leveling operation of the system, i.e., the flow of power to or from the batteries, the motor-generator and the drive system. These motor-generator speed variations depend significantly upon the characteristic of a free running internal combustion engine to respond naturally to changes in shaft loading by changes in engine speed, i.e., the engine slows down under load and speeds up as the load is reduced. Because the engine and motor-generator are directly connected, changes in the engine speed, caused either by road load variations or by other drive system demands, result in corresponding changes in the speed and power output of the motor-generator.
In the generator mode, the power supplied to the batteries by the motor-generator increases the more the speed exceeds the no-load speed. However, the motor-generator also places a load on the internal combustion engine. This load slows the rotation of the internal combustion engine and tends to hold the operating point close to the preferred speed. By contradiction, in the motor mode the power supplied by the motor-generator tends to accelerate the rotation of the internal combustion engine. In the heavy load situation, where the motor-generator is in the motor mode, this added power also pushes the internal combustion engine closer to its preferred operating speed.
By way of example, when a vehicle equipped with a hybrid engine system, which is traveling along powered by the internal engine alone, encounters a hill, the increased load caused by ascending the hill causes the vehicle and the internal combustion engine to slow down. When the internal combustion engine slows below its preferred speed, which is also the no-load speed of the motor-generator, the motor-generator functions as a motor. The motor-generator acting as a motor transfers energy from the storage battery to the drive shaft, increasing the speed of the vehicle and, hence, increasing the speed of the internal combustion engine back toward its preferred speed. Conversely, when the vehicle starts descending a hill or slowing for a traffic light, the internal combustion engine, having less load, will begin operating above its preferred speed. The motor-generator then is similarly driven above its no-load speed and functions as a generator. In the generator mode, the motor-generator puts a load on the internal combustion engine which again brings its speed back toward the preferred speed and at the same time converts any excess engine power into electric energy to be stored in the batteries. The response of the system is, of course, so smooth and continuous that the motor-generator will change modes without the average operator noticing any change in the internal combustion engine performance. Thus, both power units run simultaneously and load share, all the time conserving energy without elaborate controls.